1. Field of the Invention
The present invention relates to a magnetoresistive effect element, and a magnetic head and a magnetic reproducing apparatus including the same.
2. Description of the Related Art
In recent years, the development of a magnetoresistive effect element has been progressing. In particular, in accordance with the development of a magnetoresistive effect film exhibiting a giant magnetoresistive effect (GMR), a magnetic device including the same, especially a magnetoresistive effect head (MR head) used as a magnetic head has made a remarkable improvement in its performance.
A spin-valve film is a known example of the GMR film. The spin-valve film has a nonmagnetic layer (called an intermediate layer, a spacer layer, or the like) interposed between two ferromagnetic layers. In such a structure, an exchange bias magnetic field is applied to one of the ferromagnetic layers (called a magnetization fixed layer, a pinned layer, or the like) so as to fix its magnetization, and the magnetization of the other ferromagnetic layer (called a magnetization free layer, a free layer, or the like) is inverted by an external magnetic field such as a signal magnetic field. Then, a relative angle of magnetization directions of the magnetization fixed layer and the magnetization free layer is changed, so that a high magnetoresistive effect can be obtained.
As a GMR element, there has been proposed an element having a so-called CPP (Current Perpendicular to Plane) structure in which a sense current is passed in a direction perpendicular to a film plane of a magnetoresistive effect film. In a spin-valve GMR element, an improvement in a MR change ratio by applying the CPP structure is also expected, and it has been reported that the CPP-GMR element has achieved a MR change ratio about ten times as large as that of a CIP (Current in Plane) GMR element. Further, the CPP-GMR element also has an advantage of having a lower resistance compared with that of a TMR element utilizing a tunnel effect.
In the spin-valve GMR element, the total thickness of spin-dependent layers is very small and the number of interfaces is also small. Therefore, when the CPP structure is applied thereto, the resistance of the element becomes small, resulting in a small output absolute value. A structure under consideration in order to increase the resistance of the element is to connect the magnetization fixed layer and the magnetization free layer by a microscopic conduction part (metal path). It has been known that magnetic coupling between the magnetization fixed layer and the magnetization free layer can be broken even when such a conduction part is made of magnetic metal, if a two-dimensional size of the conduction part is of an atomic order to several nm order (see Phys. Rev. Let. 83, 2425 (1999)). In addition, it has been reported that very large resistance change ratio such as 3000% can be obtained in a joint portion (point contact) made of such a conduction part (see Phys. Rev. B, 66 020403R (2002)).
There are various opinions on why the aforesaid large resistance change ratio is achieved by the point contact, and according to one of the opinions, conductance quantization in the joint portion is thought to be involved. A GMR element utilizing such a point contact structure is disclosed in, for example, Japanese Patent Laid-open Application No. 2003-204095. This patent document describes a spin-valve GMR element in which an insulation layer disposed between a magnetization fixed layer and a magnetization free layer has an opening whose maximum width is 20 nm or smaller, and a ferromagnetic substance is filled in this opening to form a magnetic micro contact. Here, the micro opening is formed in the insulation layer with a needle or the like, and the ferromagnetic substance is deposited on the insulation layer including this opening to form the magnetic micro contact.
The conventional spin-valve GMR element using the point contact structure has a problem that sufficient enhancement of the crystallinity of the conduction part cannot be achieved since the conduction part is formed by filling the ferromagnetic substance in the micro opening formed in the insulating film. This is a factor obstructing the conductance quantization in the conduction part. Further, in the vicinity of the insulation layer (intermediate layer) having the conduction part, a current flows toward the conduction part, and therefore, control over the state of a current path in the ferromagnetic layer including the conduction part is necessary in order to promote the conductance quantization. However, in the conventional spin-valve GMR element with the CPP structure, controlling the state of the current path in the ferromagnetic layer has not been achieved yet.